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Inverse-Designed Diamond Photonics
Constantin Dory, Dries Vercruysse, Ki Youl Yang, Neil V. Sapra, Alison E. Rugar, Shuo Sun, Daniil M. Lukin, Alexander Y. Piggott, Jingyuan L. Zhang, Marina Radulaski, Konstantinos G. Lagoudakis, Logan Su, Jelena Vuckovic
TL;DR
Diamond quantum photonic circuits are limited by fabrication geometries that restrict device functionality and integration. This paper uses inverse design to create compact, fabricable interfaces and integrate them with cavities and waveguide-splitters. The resulting devices show high efficiency, robustness, and circuit-level integration.
Problem
Diamond fabrication constraints restrict device geometries and functionality, limiting integrated quantum photonic circuits.
Method
The work uses optimization-based inverse design to search fabricable device parameter spaces and develop vertical couplers and integrated diamond photonic circuits.
Results
The fabricated couplers achieve 24.2% average efficiency across 30 devices, while vertical-coupler interfaces produce a greater-than-550-fold cavity count-rate increase over notch interfaces.
Takeaways & Limitations
Inverse-designed diamond devices provide efficient, compact, and robust building blocks for more complex integrated quantum circuits.
Takeaways & Limitations
The vertical couplers have a theoretical maximum coupling efficiency of 50% because z-axis symmetry couples light equally in both directions.
Abstract
from arXiv · showhide
Diamond hosts optically active color centers with great promise in quantum computation, networking, and sensing. Realization of such applications is contingent upon the integration of color centers into photonic circuits. However, current diamond quantum optics experiments are restricted to single devices and few quantum emitters because fabrication constraints limit device functionalities, thus precluding color center integrated photonic circuits. In this work, we utilize inverse design methods to overcome constraints of cutting-edge diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications. Our design method leverages advanced optimization techniques to search the full parameter space for fabricable device designs. We experimentally demonstrate inverse-designed photonic free-space interfaces as well as their scalable integration with two vastly different devices: classical photonic crystal cavities and inverse-designed waveguide-splitters. The multi-device integration capability and performance of our inverse-designed diamond platform represents a critical advancement toward integrated diamond quantum optical circuits.
Inverse-Designed Diamond Photonics
Diamond’s optical properties and color centers motivate integrated photonic circuits, but fabrication constraints restrict device geometries and functionality. The work addresses these challenges with inverse design and demonstrates fabricable diamond devices and circuit integration.
- Diamond’s material properties and color centers support applications in quantum optics, optomechanics, nonlinear optics, and quantum technologies.
- Limited high-quality diamond thin films and nonrepeatable thinning constrain photonic crystal cavity fabrication.
- Angled etching of bulk diamond produces triangular cross-sections with strongly constrained geometries that limit device design and functionality.
- Quasi-isotropic etching enables rectangular diamond membranes with variable dimensions but introduces feature-size constraints linked to etch depth and undercut.
- Inverse design searches the full parameter space of fabricable devices to address diamond fabrication and design challenges.
- The work demonstrates a compact vertical coupler and integrates inverse-designed couplers and waveguide-splitters with diamond photonic circuits.
Inverse design of diamond nanophotonic devices
The paper develops inverse-designed diamond vertical couplers that replace low-efficiency notches while respecting fabrication constraints. The approach combines electromagnetic optimization with fabrication penalties and supports compact, selective free-space-to-waveguide coupling.
- Existing diamond platforms cannot readily use common grating-coupler strategies, while hybrid GaP-on-diamond structures confine the optical field outside diamond emitters.
- Notches provide a practical baseline but have approximately 1% scattering efficiency.
- The inverse-designed vertical coupler occupies 1.0 × 1.0 μm2 and couples directly to a 400 nm waveguide without a taper.
- ≈25% simulated peak efficiency for the coupler versus ≈1% for the notch demonstrates the inverse-designed interface’s performance advantage.
- The coupler targets selective coupling between the free-space TEM00 mode and the waveguide’s fundamental TE mode.
- Theoretical coupling efficiency is limited to 50% because z-axis symmetry sends light equally in the +z and −z directions.
- The optimization maximizes incident-light coupling into the fundamental TE mode while transitioning from continuous permittivity to discrete, fabricable structures.
- Fabrication penalties reduce the 737 nm coupling-efficiency peak from ≈27.5% to ≈25% while enforcing fabricable features.
Characterization of diamond vertical couplers
The vertical couplers were experimentally characterized for polarization response, coupling efficiency, bandwidth, and robustness across fabricated devices.
- Polarization and efficiency: Rotating the input polarization produced a five-fold reduction in transmitted power, consistent with simulations and coupling to a linearly polarized TEM00 mode.The measurement provides experimental evidence for the intended polarization-selective free-space interface.
- Measurement approach: The characterization used top-down and side-view measurements, optical microscopy, polarization sweeps, and fiber-collected transmission spectra.The device geometry and measurement workflow are illustrated in the cited figure passages.
- Polarization and efficiency: Peak coupling efficiencies reached ≈21 % with a polarization-maintaining fiber and ≈26.5 % with a multimode fiber.The measured bandwidth exceeded 70 nm for the polarization-maintaining fiber and 90 nm for the multimode fiber.
- Polarization and efficiency: The measured efficiencies agreed well with numerical simulations, while the small fiber-dependent discrepancy indicated efficient coupling into the fundamental free-space mode TEM00.The comparison used both polarization-maintaining and multimode fiber collection.
- Robustness: Fifteen devices measured under coarse alignment showed robust transmission, and 30 fabricated devices averaged 24.2 % efficiency across varied doses.The optimization imposed minimum feature sizes to support fabrication yield and robustness.
Diamond quantum optical interfaces
The compact vertical couplers provide a free-space interface compatible with diamond nanophotonic resonators, substantially outperforming notches in cavity readout measurements.
- Interface design: The vertical coupler is compact, robust, and designed for simultaneous fabrication with high-Q/V resonators without additional fabrication steps.It is intended as a free-space interface for cavity quantum electrodynamics and avoids steps that could affect resonator performance.
- Cavity characterization: Nanobeam photonic crystal cavities were measured by coupling a free-space laser into their TE fundamental mode and then into the cavity modes.The comparison included devices using inverse-designed vertical couplers and notches as free-space interfaces.
- Performance comparison: > 550-fold higher cavity-resonance counts were measured with the vertical coupler than with the notch device at comparable Q ≈4000.The measured enhancement matched the 625-fold enhancement expected from simulations.
- Performance comparison: The cavity-to-waveguide extraction efficiency was ≈24 times greater for a vertical coupler than for a notch.This result was obtained by directly coupling to the cavity and optimizing collection from each interface.
- Experimental consequence: The higher coupling efficiency reduced some photon-integration times from weeks to minutes.The passage presents this reduction as an experimental consequence of the improved interface performance.
Inverse-designed diamond photonic circuit
Inverse-designed components were integrated into a diamond circuit combining vertical couplers, waveguide-splitters, and nanobeam cavities for multi-cavity optical experiments.
- Circuit requirements: Scaling quantum photonics requires excitation of multiple cavities, interference on beamsplitters, and efficient photon collection and detection.The passage identifies waveguide-splitters as difficult to realize in suspended diamond using angled-etch fabrication.
- Integrated circuit: The fabricated conceptual circuit combines vertical couplers, waveguide-splitters, and nanobeam photonic crystal cavities with different geometries.The two cavity outputs are interfered and routed through the circuit for off-chip coupling.
- Integrated circuit: The inverse-designed waveguide-splitter was designed for a 50:50 splitting ratio with simulated efficiencies of 95 %.The two nanobeam resonances were detuned by < 1 nm because of fabrication imperfections and tuned using gas condensation.
- Implications: The circuit demonstrates inverse design overcoming limitations of classical photonics for large-scale on-chip quantum optics experiments.The supported extension directions include greater compactness, arbitrary emitter locations, phase matching, and bandwidth-specific optimization.
Highly efficient free-space-waveguide interfaces
The study projects higher-efficiency free-space interfaces by relaxing fabrication constraints, breaking vertical symmetry, and adding reflective structures.
- Efficiency targets: Efficiencies above 90 % are achievable with fiber tapers, but their substantially larger footprints motivate compact alternatives.The study compares this trade-off with inverse-designed free-space-waveguide interfaces.
- Reflective enhancement: Suspended-air devices reach 67.9 % efficiency, while aluminum back-reflectors raise efficiencies to 72.4 % on SiO2 and 86.6 % in suspended structures.The back-reflector distances are optimized relative to the coupler to match reflected and directly coupled photon phases.
- Application scope: These designs are presented as compact alternatives to fiber tapers for single-photon quantum photonic applications.The same interfaces may support optical driving of individual emitters for Raman-based compensation of spectral broadening.
Discussion
The work uses optimization-based inverse design to address diamond nanofabrication constraints and develop efficient building blocks for diamond nanophotonic circuits. Optical free-space couplers and a small diamond photonic circuit achieve high efficiency, compactness, and robustness.
- Optimization-based inverse design addresses constraints from cutting-edge diamond nanofabrication.
- Optical free-space couplers and a small diamond photonic circuit achieve high efficiency, compactness, and robustness.
- These compact building blocks enable more complex quantum circuits with varied device components.